Abstract:
A "machine" is any causal physical system, hence we are machines,
hence machines can be conscious (feel). The question is: which
*kinds* of machines can feel, and how? Chances are that robots that
can pass the Turing Test -- completely indistinguishable from us in
their behavioral capacities -- can feel, but we can never be sure
(because of the "other-minds" problem). And we can never explain or
understand *how* or *why* they manage to feel, if they do, because
of the "mind/body" problem. We can only know how they pass the
Turing Test. That is why this problem is not just *hard* -- it is
insoluble.

Asking whether a machine
can be conscious is rather like asking whether one has stopped beating one's
wife: The question is so heavy with assumptions that either answer would
be incriminating!

The answer,
of course, is: It depends entirely on what you mean by “machine”! If you
mean the current generation of man-made devices (toasters, ovens, cars,
computers, today’s robots), the answer is: almost certainly not.

Empirical
Risk. Why "almost"? Two reasons, the first being
the usual one: (1) “empirical risk.” We know since at least Descartes that
even scientific "laws" are merely very probable, not certain. Only mathematical
laws -- which describe consequences that follow provably (i.e., on pain
of contradiction) from our own assumptions -- are necessarily true.
But this certainty and necessity are unnecessary for physics; almost-certainty
will do. "Can a particle travel faster than the speed of light?" Almost
certainly not (at least on current-best theory -- or at least the last word
of it that trickled down to this non-physicist). "Could a particle
travel faster than light?" That certainly is not provably impossible, though
it might be impossible given certain assumptions. (But those assumptions
are not necessarily correct.)

The Other-Minds Problem.
Empirical risk besets all scientific hypotheses, but let us agree that it
is not something we will worry about here. There is no need for roboticists
to be holier than physicists. The second reason for the "almost" is peculiar
to robotics, however, and it is called (2) the "other minds problem” (Harnad
1991): There is no way to be certain that any other entity than myself is
conscious (I am speaking deictically: please substitute yourself for me,
if you too are conscious). This too we owe to Descartes.

In the long
history of philosophy the other-minds problem has been puzzled over for
a variety of reasons, usually variants on questions about what one can and
cannot know for sure. These epistemic questions are interesting, but we will
not worry about them here, for the usual reason, which is the following:
It looks on the face of it as if the right strategy for handling the other-minds
problem is identical to the strategy for handling empirical risk, namely,
to note that although we can only be 100% certain about two things -- about (1) mathematics and about (2) our own consciousness
-- all else being just a matter of probability, some things, such as scientific
laws and the consciousness of our fellow-human beings, are nevertheless
so close to 100% sure that it is a waste of time worrying about them. (Let
us also note, though, that in the empirical science of robotics, that extra
layer of risk that comes from the other-minds problem might just come back
to haunt us.)

So let us agree not to
worry about the other-minds problem for now: People other than myself are
almost certainly conscious too, and toasters and all other human artifacts
to date are almost certainly not.

What Is a Machine?
Have we now answered the question "Can a machine be conscious?" It sounds
as if, at the very least, we have answered the question "Is any machine
we have built to date conscious?" That makes the original question sound
as if it was only asking about what we can and cannot build, which is like
asking whether we can build a rocket that can reach Alpha Centauri (a rather
vague and arbitrary question about quantitative limitations on future technology).
But is "machine" defined as "what human beings can build"? I think
that defines "artifact" -- but "machine"? Or rather, do we really want to
ask merely: "Can a man-made artifact be conscious?"

And even that would be
rather vague, for "man-made" is itself rather vague. Common sense dictates
that human procreation does not count as "man-making" in this context. But
what about genetic or other biological engineering? If the day comes when
we can craft organisms, even humans, molecule by molecule, in the laboratory,
does anyone -- or rather, anyone who has agreed to discount the other-minds
problem when it comes to naturally crafted fellow-humans -- doubt that such
a bottom-up construction of a clone would be conscious too?

So "man-made" is a wishy-washy
term. It does not pick out what we mean by "machine" here. Surely a toaster
(that very same device) would not become more eligible for consciousness
if it happened to grow on a tree instead of being fabricated by one of us.
By the same token, a toaster would not become any less of a "machine"
(whatever that turns out to mean) by growing on a tree: Two toasters, identical
right down to the last component, one of which I built and the other of
which grew on a tree, are surely both "machines" (whatever that means) if
either one of them is. Another way to put this is that we need a definition
of "machine" that is strictly structural/functional, and not simply dependent
on its historic origins, if we want to make our question about what machines
can and cannot do (or be) into a substantive rather than an arbitrary one.

Kinds of
Machines. But I am afraid that if we do follow this much
more sensible route to the definition of "machine," we will find that a
machine turns out to be simply: any causal physical system, any "mechanism."
And in that case, biological organisms are machines too, and the answer
to our question "Can a machine be conscious" is a trivial "Yes, of course."
We are conscious machines. Hence machines can obviously be conscious.
The rest is just about what kinds of machines can and cannot be
conscious, and how -- and that becomes a standard empirical research
program in "cognitive science": The engineering side of cognitive science
would be the forward-engineering of man-made conscious systems and the biological
side of cognitive science would be the reverse-engineering of natural conscious
systems (like ourselves, and our fellow-organisms): figuring out how our
brains work.

Except for one problem,
and it is the one that risked coming back to haunt us: What does it mean
to "forward-engineer" (or, for that matter, to "reverse-engineer") a conscious
system? It is to give a causal explanation of it, to describe fully the inner
workings of the mechanism that gives rise to the consciousness.

Forward-
and Reverse-Engineering the Heart and Brain. Let us take
a less problematic example: To forward-engineer a cardiac system (a heart)
is to build a mechanism that can do what the heart can do.
To reverse-engineer the heart is to do the same thing, but in such a way
as to explain the structure and the function of the biological heart itself,
and not merely create a prosthesis that can take over some of its function.
Either way, the explanation is a structural/functional one. That is, both
forward and reverse engineering explain everything that a heart can do,
and how, whereas reverse engineering goes on to explain what the heart is
(made out of), and how it in particular happens to do what hearts can do.

Now let us try to carry
this over to the brain, which is presumably the organ of consciousness.
To forward-engineer the brain is to build a mechanism that can do what the
brain can do; to reverse engineer the brain is to do the same thing, but
in such a way as to explain the structure and function of the biological
brain itself. Either way, the explanation is a structural/functional one.
That is, both forward and reverse engineering explain everything that a brain
can do, and how, whereas reverse engineering goes on to explain what
the brain is (made out of), and how it in particular happens to do
what brains can do: how it works.

How does the ghost of the
other-minds problem spoil this seemingly straightforward extension of the
cardiac into the cogitative? First, consider the forward-engineering: If
we were forward-engineering cardiac function, trying to build a prosthesis
that took over doing all the things the heart does, we would do it by continuing
to add and refine functions until we eventually built something that was
functionally indistinguishable from a heart. (One test of our success might
be whether such a prosthesis could be implanted into humans from cradle to
grave with no symptom that it was missing any vital cardiac function.) This
forward-engineered cardiac system would still be structurally distinguishable
from a natural heart, because it had omitted other properties of the heart
-- noncardiac ones, but biological properties nonetheless -- and to capture
those too may require reverse-engineering of the constructive, molecular kind
we mentioned earlier: building it bottom-up out of biological components.

The thing to note is that
this cardiac research program is completely unproblematic. If a vitalist had
asked "Can a machine be cardiac?" we could have given him the sermon about
"machines" that we began with (i.e., you should instead be asking “What
kind of machine can and cannot be cardiac, and how?”). Next we could
have led him on through forward-engineering to the compleat reverse-engineered
heart, our constructed cardiac clone, using mechanistic principles (i.e.,
structure, function, and causality) alone. At no point would the cardiac
vitalist have any basis for saying: "But how do we know that this machine
is really cardiac?" There is no way left (other than ordinary empirical
risk) for any difference even to be defined , because every structural
and functional difference has been eliminated in the compleat reverse-engineered
heart.

The same would be true
if it had been life itself and not just cardiac function that had been
at issue: If our question had been "Can a machine be alive?" the very same
line of reasoning would show that there is absolutely no reason to doubt
it (apart from the usual empirical risk, plus perhaps some intellectual or
technological doubts of the Alpha Centauri sort). Again, the critical point
is when we ask of the man-made, reverse-engineered clone: "But how do we
know that this machine is really alive?" If there are two structurally
and functionally indistinguishable systems, one natural and the other man-made,
and their full causal mechanism is known and understood, what does it even
mean to ask "But what if one of them is really alive, but the other
is not?" What property is at issue that one has and the other lacks, when
all empirical properties have already been captured by the engineering (Harnad
1994)?

The Animism at the Heart
of Vitalism. Yet this last worry -- "How can we know it's alive?" --
should sound familiar. It sounds like the other-minds problem. Indeed, I
suspect that, if we reflect on it, we will realize that it is the other-minds
problem, and that what we are really worrying about in the case of the man-made
system is that there's nobody home in there, there is no ghost in the machine,
And that ghost, as usual, is consciousness. That's the property that we are
worried might be missing.

So chances are that it
was always animism that was at the heart of vitalism. Let us agree to set
vitalism aside, however, as there is certainly no way we can know whether
something can be alive yet not conscious (or incapable of returning to consciousness).
Plants and micro-organisms and irreversibly comatose patients will always
be puzzles to us in that respect. So let us not dwell on these inscrutable
cases and states. Logic already dictates that any vitalist who does accept
that plants are not conscious would be in exactly the same untenable
position if he went on to express scepticism about whether the compleat
artificial plant is really alive as the sceptic about the compleat
artificial heart (worried about whether it's really a heart): If there's
a difference, what's the difference? What vital property is at issue? If
you can't find one (having renounced on consciousness itself), then you are
defending an empty distinction.

Turing-Testing. But
the same is most definitely not true in the case of worries about
consciousness itself. Let us take it by steps. First we forward-engineer
the brain: We build a robot that can pass the Turing Test (Turing 1950; Harnad
1992): It can do everything a real human can do, for a lifetime, indistinguishably
from a real human (except perhaps for appearance: we will return to that).

Let us note, though, that
this first step amounts to a tall order, probably taller than the order of
getting to Alpha Centauri. But we are talking about "can" here, that is,
about what is possible or impossible (for a machine), and how and why, rather
than just what happens to be within our actual human technological reach.

Doing vs. Feeling: The
Feeling/Function Problem. So supposing we do succeed in building such
a Turing-scale robot (we are no longer talking about toasters here). Now,
the question is whether he is really conscious: On the face of it, the only
respect in which he is really indistinguishable from us is in everything
he can do. But conscious is something I am, not something I
do. In particular, it is something I feel; indeed, it is the
fact that I feel. So when the sceptic about that robot's consciousness
-- remember that he cannot be a sceptic about machine consciousness in general:
we have already eliminated that by noting that people are a kind of machine
too -- wants to say that that robot is the wrong kind of machine,
that he lacks something essential that we humans have, we all know exactly
what difference the sceptic is talking about, and it certainly is not an
empty difference. He is saying that the robot does not feel, it merely
behaves -- behaves exactly, indeed Turing-indistinguishably -- as if it feels,
but without feeling a thing.

Empirical Robotics.
It is time to remind ourselves of why it is that we agreed to set aside
the other-minds problem in the case of our fellow-human beings: Why is it
that we agreed not to fret over whether other people really have minds (as
opposed to merely acting just as if they had minds, but in reality being
feelingless Zombies)? It was for the same kind of reason that we don't worry
about empirical risk: Yes, it could be that the lawful regularities that
nature seems to obey are just temporary or misleading; there is no way to
prove that tomorrow will be like today; there is no way to guarantee that
things are as they appear. But there is no way to act on the contrary either
(as long as the empirical regularities keep holding).

Empirical risk is only
useful and informative where there is still actual uncertainty about the regularities
themselves: where it is not yet clear whether nature is behaving as if it
is obeying this law or that law; while we are still trying
to build a causal explanation. Once that is accomplished, and all appearances
are consistently supporting this law rather than that one, then fretting
about the possibility that despite all appearances things might be otherwise
is a rather empty exercise. It is fretting about a difference that makes
no difference.

Of course, as philosophers
are fond of pointing out, our question about whether or not our Turing robot
feels is (or ought to be) an ontic question -- about what really
is and is not true, what really does or does not, can or cannot, exist --
rather than merely an epistemic question about what we can and cannot
know, what is and is not "useful and informative," what does or does not
make an empirical difference to us. Epistemic factors (what's knowable or
useful to know) have absolutely no power over ontic ones (what there is,
what is true).

It would be wise for mere cognitive scientists to concede this point. Just
as it is impossible to be certain that the laws in accordance with which
nature seems to behave are indeed the true laws of nature, it is impossible
to be certain that systems that behave as if they feel, truly feel.
Having conceded this point regarding certainty, however, only a fool
argues with the Turing-Indistinguishable: Yes, the true laws could be other
than the apparent laws, but if I can't tell the two apart empirically, I'd
best not try to make too much of that distinction! By the same token, a robot
that is indistinguishable for a lifetime from a feeling person might be a
Zombie, but if I can't tell the two apart empirically, I'd best not try to
make too much of that distinction (Harnad 2000).

Indistinguishable? But
surely there are plenty of ways to distinguish a robot from a human being.
If you prick us, do we not bleed? So perhaps the sceptic about the forward-engineered
robot should hold out for the reverse-bio-engineered one, the one made out
of the right stuff, Turing-indistinguishable both inside and out, and at
both the macro and micro levels. It is only about that machine that we can
reply to our reformulated question -- "What kinds of machine can and cannot
be conscious?” -- that only that kind can.

But would we be right,
or even empirically or logically justified in concluding that? To put it
in a more evocative way, to highlight the paradoxical polarity of the "risks"
involved: Would we be morally justified in concluding that whereas the reverse-bioengineered
machines, because they are empirically indistinguishable from natural machines
like us, clearly cannot be denied the same human rights as the rest of us,
the forward-engineered machines, because they are merely Turing-indistinguishable
from us in their (lifelong) behavioral capacity can be safely denied
those rights and treated as unfeeling Zombies (toasters)?

Other-Mind Reading and
Turing-Testing.To answer this question we need to look a little more
closely at both our empirical methodology in cognitive science and our moral
criteria in real life. Let us consider the second first. Since at least
1978 (Premack 1978) there has grown an area of research on what is sometimes
called "theory of mind" and sometimes "mind-reading," in animals and children
. This work is not a branch of philosophy or parapsychology as it might sound;
it is the study of the capacity of animals and children to detect or infer
what others "have in mind." (As such, it should really be called research
on "other-mind perception.")

It has been found that
children after a certain age, and certain animals, have considerable skill
in detecting or inferring what others (usually members of their own species)
are feeling and thinking (Whiten 1991; Baron-Cohen
1995). The propensity for developing
and exercising this mind-reading skill was probably selected for by evolution,
hence is inborn, but it also requires learning and experience to develop.
An example of its more innate side might be the capacity to understand facial
expressions, gestures and vocalizations that signal emotions or intentions
such as anger and aggression; a more learning-dependent example might be the
capacity to detect that another individual has seen something, or wants something,
or knows something.

Let us note right away
that this sort of mind-reading is a form of Turing-testing: inferring mental
states from behavior. The "behavior" might be both emitted and detected completely
unconsciously, as in the case of the release and detection of pheromones,
or it might be based on very particular conscious experiences such as when
I notice that you always purse your lips in a certain way when you think
I have lied to you. And there is everything in between; my sense of when
you are agitated vs. contented, along with their likely behavioral consequences,
might be a representative midpoint. Language (which, let us not forget,
is also a behavior) is probably the most powerful and direct means of mind-reading
(Harnad 1990; Cangelosi & Harnad 2000).

Hence, apart perhaps from
direct chemical communication between brains, all mind-reading is based on
behavior: Turing-testing. It could hardly have been otherwise. We know, again
since at least Descartes, that the only mind we can read other than
by Turing-testing is our own!As far as all other
minds are concerned, absent genuine telepathic powers (which I take to be
a fiction, if not incoherent), the only database available to us for other-mind-reading
is other-bodies' behavior.

We do have to be careful not to make the ontic/epistemic conflation here:
The foregoing does not mean that all there is to mind is behavior (as the
blinkered behaviorists thought)! But it does mean that the only way to read
others' minds is through their behavior, i.e., through Turing-testing.

Functional- vs. Structural/Functional-Indistinguishability.
Now, back to our two robots, the reverse-bioengineered one to whom we were
ready to grant human rights and the merely forward-engineered one about
whom we were not sure: Both are Turing-indistinguishable from us behaviorally,
but only the first is anatomically correct. We're all machines. Is only
the first one the right kind of machine to have a mind? On what basis
could we possibly conclude that? We have ascertained that all mind-reading
is just behavior-based Turing-testing, and all three of us (the two man-made
robots and me) are indistinguishable in that respect. What else is there?
The rest of the neuromolecular facts about the brain? Which facts?

There are
countless facts about the brain that could not possibly be relevant to
the fact that it has a mind: its weight, for example. We know this, because
there is a huge range of variation in human brain mass -- from the massive
brain of a huge man to the minute brain of a microcephalic, who nevertheless
feels pain when he is pinched. Now imagine trying to narrow down the properties
of the brain to those that are necessary and sufficient for its having a
mind. This turns out to be just another variant of our original question:
"What kinds of machines can and cannot be conscious?" We know brains can
be, but how? What are their relevant properties (if their weight, for example,
is not)? Now imagine paring down the properties of the brain, perhaps by
experimenting with bioengineered variations, in order to test which ones
are and are not needed to be conscious. What would the test be?

Turing-Filtering Relevant
Brain Function. We are right back to Turing-testing again! The only
way to sort out the relevant and irrelevant properties of the biological
brain, insofar as consciousness is concerned, is by looking at the brain's
behavior. That is the only non-telepathic methodology available to us, because
of the other-minds problem. The temptation is to think that "correlations"
will somehow guide us: Use brain imaging to find the areas and activities
that covary with conscious states, and those will be the necessary and sufficient
conditions of consciousness. But how did we identify those correlates? Because
they were correlates of behavior. To put it another way: When we ask a human
being (or a reverse-bioengineered robot) "Do you feel this?" we believe that
the accompanying pattern of activity is conscious because we believe him
when he says (or acts as if) he feels something -- not the other way round:
It is not that we conclude that his behavior is conscious because of the
pattern of brain activity; we conclude that the brain activity is conscious
because of the behavior.

So, by the same token,
what are we to conclude when the forward-engineered robot says the same
thing, and acts exactly the same way (across a lifetime)? If we rely on the
Turing criterion in the one case and not the other, what is our basis for
that methodological (and moral) distinction? What do we use in its place,
to conclude that this time the internal correlates of the very same behavior
are not conscious states?

The answer to our revised
question -- "What kinds of machines can be conscious (and how)?" has now
come into methodological focus.The answer is: The
kinds that can pass the Turing Test, and by whatever means are necessary and
sufficient to pass the Turing Test.

Darwin and Telepathy.
If we have any residual worries about Zombies passing the Turing Test, there
are two ways to console ourselves. One is to remind ourselves that not even
the Blind Watchmaker who forward-engineered us had a better way: survival
and reproduction are just Turing functions too: Darwin is no more capable
of telepathy than we are. So there is no more (or less) reason to worry that
Zombies could slip through the Turing filter of evolution than that they
could slip through the Turing filter of robotic engineering (Harnad 2002).

Turing and Telekinesis.
Our second consolation is the realization that the problem of explaining
how (and why) we are not Zombies (Harnad 1995) (otherwise known as
the “mind/body problem”) is a “hard” problem (Shear 1997), and not one we
are ever likely to solve. It would be easy if telekinetic powers existed:
Then feelings would be physical forces like everything else. But there is
no evidence at all that feelings are causal forces. That is why our forward-
and reverse-engineering can only explain how it is that we can do things,
not how it is that we can feel things. And that is why the ghost in
the machine is destined to continue to haunt us even after all cognitive
science’s empirical work is done (Harnad 2001).